To improve propelling efficiency in existing boats, there have been proposed a technique of providing an auxiliary propeller or propelling device such as an AT fin (Addition Thruster Fin) at the rudder behind an ordinary propeller, or the technique of providing an auxiliary propeller or propelling device such as a PBCF (Propeller Boss Cup Fin) immediately behind the propeller to improve the efficiency of the propeller itself. In the latter technique, the axial space associated with the ordinary propeller shaft is very limited, and therefore it is necessary to enlarge the propeller shaft or to provide a new propeller shaft.